This invention relates to television pick-up tubes and particularly to an improved mesh assembly having reduced microphonics for such a tube.
Microphonics is a physical vibration of some portion of a tube which, because of its movement, produces an undesirable electrical signal output or noise which manifests itself as a background of lines or striations in a television picture.
In a vidicon type pick-up tube, a mesh grid is disposed between a photoconductive target and an electron gun which provides a scanning electron beam. The mesh provides a lens action which causes the electron beam from the electron gun to impinge perpendicularly on the target electrode. The mesh is usually supported around its periphery by at least one annular support ring.
When such tubes are subjected to mechanical shock and vibration, for example, from cooling fans or incident sound from external sources, the conductive mesh will start vibrating relative to the photoconductive target and will cause microphonic effects which produce the above-described background lines and striations in the picture.
Many expedients such as rectangularly-shaped damping members in contact with the mesh have been adopted to eliminate the undesirable microphonic effects. Such configurations provide a rectangularly-shaped raster format and thus limit the orientation of the tube within a multitube color camera thereby complicating the tube alignment procedure. Among the other expedients adopted is a structure described in U.S. Pat. No. 3,906,278 to Horton et al., issued Sept. 16, 1975 comprising a fine flexible mesh washer clamped between two annular members. The annular members are dished away from each other beyond the region of clamping. The flexible mesh washer extends inwardly beyond the region of clamping and contacts both the fine conductive mesh and the dished portion of one of the clamping members in order to damp vibrations. The Horton et al. structure is complex and requires a precisely-formed flexible mesh washer. If the corrugations of the mesh washer are too shallow, the washer will not contact both the dished annular member and the fine mesh. If this occurs, little or no damping will occur. Being constructed from mesh, the flexible washer also tends to undergo a change in elasticity after repeated thermal and mechanical cycling thus decreasing the effectiveness of the damping action.
A simple mesh damping structure is described in U.S. Pat. No. 4,323,814 issued to Benner et al. on Apr. 6, 1982, and entitled, "MESH ASSEMBLY HAVING REDUCED MICROPHONICS FOR A PICK-UP TUBE". The Benner et al. patent is assigned to the same assignee as the present invention and is incorporated by reference herein for disclosure purposes.
The Benner et al. structure comprises a mesh electrode disposed between a frustro-conically-shaped mesh support ring and a dished, i.e., frustro-conically-shaped, spring-like mesh damping ring. The mesh damping ring is compressed into a reversal of its dished shape and welded at a plurality of points to the outer periphery of the mesh support ring. By reversing the dished shape of the mesh damping ring, the damping ring assumes an undulatory or serpentine configuration which contacts the mesh electrode periodically around the inner periphery of the damping ring. The area of contact extends radially outward from the inner periphery to the weld points at the outer periphery of the damping ring.
An improved mesh damping structure is described in U.S. Pat. No. 4,347,459 issued to T. E. Benner on Aug. 31, 1982 and entitled, "IMPROVED MESH ASSEMBLY HAVING REDUCED MICROPHONICS FOR A PICK-UP TUBE". The Benner patent; and is assigned to the same assignee as the present invention and is incorporated by reference herein for disclosure purposes.
The Benner structure comprises a mesh electrode disposed between an annular support ring and an annular damping ring having a substantially bow-shaped cross-section extending from the inner periphery to the outer periphery thereof. A plurality of arcuately-shaped regions are formed into the inner periphery of the damping ring so that when the damping ring contacts the mesh electrode, a vibration damping chamber is formed in the annular region between the inner and outer periphery of the bow-shaped damping ring and the axially-extending region between the damping ring and the mesh electrode.
Both the Benner et al. and the Benner structures disclosed in the above-indicated patents require properly tempered, accurately manufactured mesh assemblies to ensure proper vibration damping.
A simpler mesh damping structure known in the art comprises a mesh electrode disposed between a pair of mounting rings having different inside diameters. In this structure, the frequency of the mesh vibration in one direction is different from that in the opposite direction. The frequency difference dissipates energy by destruction interference and thereby decreased vibration time. Unfortunately, it is necessary to heat the mesh and the mounting rings to a high temperature, subsequent to mounting the mesh electrode between the mounting rings, in order to clean and tighten the mesh. Frequently during the heating step, the mesh fuses to the surface of the mounting ring having the smaller inside diameter thus producing a structure that has the same vibrational frequency in each of the above-described opposite directions.